1. Field of the Invention
The present invention relates to an implantable heart stimulator of the type having a pulse generator for delivering stimulation pulses to the heart of a patient, an IEGM unit for registering electric signals associated with the patient""s heart, a classifier for classifying the received electric IEGM signals according to their waveforms in at least one segment of the cardiac cycle, and a control unit for controlling the pulse generator depending on the classification of the IEGM signals.
2. Description of the Prior Art
It is known to sense a physiologic parameter of a patient, classify the parameter, and use the information obtained for diagnostic purpose or for the purpose of treating the patient. Thus, U.S. Pat. No. 5,645,575 describes an implantable pacemaker, which senses a general physiologic parameter of the patient for use in determining the physical stress or workload of the patient. The parameter is classified and the information obtained from this classification is used to control the stimulation rate/pulse interval of the pacemaker. IEGM waveforms are not mentioned as the selected parameter to be sensed. U.S. Pat. Nos. 5,215,098 and 5,280,792 describe implantable devices, which sense IEGM signals and classify them. In the device according to U.S. Pat. No. 5,280,792 the classification results are used for adjusting the output of a cardioverter/defibrillator while the results in the device disclosed in U. S. Pat. No. 5,215,098 are used for diagnostic purposes only. In U.S. Pat. No. 5,782,885 a method and a cardiac assist system for pacing a heart of a patient are described, in which sensed IEGM signal waveforms are classified in dependence of the workload of the patient, and the results of the classification are used to control the pulse rate or stimulation interval of a pacemaker. It is also known to sense IEGM signals and classify the waveforms to use the results of this classification for identifying cardiac arrhythmias and subsequent suitable therapy, see e.g. U.S. Pat. No. 5,203,326, European Application 0 465 241 and European Application 0 653 224.
It is also known that the amplitude of the QRS complex of ordinary surface ecg:s varies with the respiration of the patient, see e.g. U.S. Pat. No. 4,757,815.
It has now been found that the waveform or morphology of IEGM:s varies with the respiration of the patient, and the present invention is based on this discovery.
An object of the invention is to provide a technique for detecting the respiration of a patient and to use the results for controlling an implantable heart stimulator.
The above object is achieved in accordance with the principles of the present invention in an implantable heart stimulator of the type initially described, wherein a filter is additionally provided to extract parameters related to the patient""s respiration from beat-to-beat variability in the IEGM signal classification, while filtering out slow classification variability extending over several cardiac cycles, and also having a control unit which receives the respiration-related parameters to control delivery of stimulation pulses from the pulse generator dependent on those parameters.
Thus the beat-to-beat variability in the IEGM signal morphology is utilized to extract respiration related parameters, while slow morphology variability extending over several cardiac cycles and related to the workload is filtered away. The heart stimulator according to the invention is consequently based merely on IEGM signal processing for realizing an accurate respiration sensor, eliminating the need for mechanical sensor components. Furthermore the technique can be easily implemented in current devices.
In an embodiment of a heart stimulator according to the invention, the classifier is formed by a neural network clustering stored IEGM signals into a predetermined number of classes depending on their waveforms. By using a neural network the classification technique used is self-organizing and adaptive and can be implemented in existing devices. The neural network forms a self-organizing feature map, which can be trained to cluster the data into a predetermined number of classes.
In another embodiment of the stimulator according to the invention, the respiration related parameters are respiration rate and respiration depth. From these two parameters the minute volume can be determined and used for controlling the heart stimulator.
In another embodiment of the heart stimulator according to the invention a second filter is connected in parallel to the aforementioned filter to extract a parameter related to the workload for the patient from slow variability, extending over several cardiac cycles, in IEGM signal classification while filtering out beat-to-beat classification variability, and the control unit is adapted to receive the workload related parameter to control the delivery of stimulation pulses from the pulse generator depending on the parameter. In this way a workload and respiration dual sensor is obtained which can be used for the control of a heart stimulator. Such a dual sensor will thus extract two different control quantities, i.e. workload and respiration, from the IEGM signal.
In a further embodiment of a heart stimulator according to the invention, the first and said second filters are high pass and low pass filtersrespectively. In this way respiration and workload parameters can be concurrently extracted from fast, beat-to-beat variability of the classification of IEGM signals and slow variability, extending over several cardiac cycles, of the classification.